Due to the environmental problems and rising oil prices, there has been an increasing demand for clean energy or new renewable energy, rather than petroleum products, which is inexpensive and generates no pollutants. In order to obtain clean energy or new renewable energy, there are many methods of generating energy using solar energy based on solar heat, wind force energy, biomass energy, hydrogen fuel energy, fuel cells, geothermal energy, marine energy, etc. Of them, the method using solar energy has been considered particularly useful as a next-generation energy production method because of the efficiency of energy generation or the degree of completion thereof. The method using solar energy is classified into two types; conversion of solar energy into electrical energy by means of a solar cell module, and conversion of solar energy into thermal energy. In particular, in the case where solar energy is converted into electrical energy using the solar cell module, the solar cell module is used outdoors for a long period of time (10 years or longer), and thus a device for protecting the solar cell module is necessary. As such, this device should essentially be able to withstand a variety of different weather conditions and be heat resistant. Further, the device is required these days to have a function capable of maximizing solar cell performance including increasing light reflectivity, in addition to having the protection function. The device for protecting the solar cell may be provided in diverse forms. Currently available is a film having weatherability and heat resistance. This film is typically disposed under the solar cell and is mainly exemplified by a polyvinyl fluoride (PVF) film containing a fluorine group having high weatherability and heat resistance. However, because the preparation cost of the PVF film is very high, various alternatives therefor have been devised or proposed.
In this regard, Japanese Unexamined Patent Publication Nos. 2008-85270, 2002-26354 and 2002-134770 disclose the use of a polyethylene terephthalate film between the PVF films, and a three-layer coextruded film composed of a film alternative for the PVF film, a film for increasing weatherability and a layer of inorganic material for increasing reflectivity. However, these films are disadvantageous because the elongation change rate thereof is measured to be about 20% in a 50-hour weathering test which is greatly required for a back-surface protective film for a solar cell, and further, is drastically decreased in a 100-hour weathering test.